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Insights: Sue Pryke on Slipcasting a Mug

In our latest DN Insight award winning ceramics designer Sue Pryke talks us through the steps in making a slipcast mug. It’s more complicated than you might think and requires considerable skill and knowledge. No wonder The Great Pottery Throwdown enlisted Sue as a judge.

Sue says: I make slipcast tableware using a coloured porcelain clay body which I make myself.  The process involves using liquid clay (also known as slip), which is poured into porous plaster moulds and allowed to thicken or set, forming a thickness on the inside of the mould; the excess is then poured out leaving behind the mug, think of an Easter egg shell.  The skill in slipcasting is a combination of understanding the thickness and fluidity of the slip and having the patience to replicate the same qualities in the casting every time.  It should be easy, shouldn’t it?

When I was in my first job in a pottery, the potter used to refer to slipcasting as cheating… back then I could see his point, most of the work is done, it’s in the mould, the rest should be easy, right! The problem is that this process has a much higher water content than other methods. The water has to be driven off at some stage which, as anyone who works with clay knows, can lead to distortion if not managed carefully.

So that’s one battle, but also the casting slip has the addition of sodium silicate which helps the particles of clay stay suspended in the water. This make it easier to pour as a slip, as it gives fluidity. If the recipe for the slip isn’t balanced this can lead to casts being too thick, too thin, too lumpy, pinholey, wreathy or just lead to warping, amongst a plethora of other unwelcome traits.

The skill lies in understanding the chemistry, the thixotropy* and being able to replicate the same recipe again and again. Managing how the slip is behaving plus reproducing the same casts with the same thickness needs patience for sure.  All of that is down to chemistry.  For me the most exciting part about slipcasting is the interaction with each piece, achieving continuity or the pristine sameness, and then the final decisions I make during the fettling process.

Fettling is the finishing of a piece before it is fired. With slipcasting the tops of pieces are trimmed flat in the mould. The job of the fettler is to round those edges off with a tool and a sponge to make the pieces ergonomic to drink from, it’s an important stage. It’s repetitive and relies on haptic skills and tacit knowledge to make final judgements about how the rim should be finished.  If the rim is too flat or too thin it’s not nice to drink from: these are important details that are usually only noticed when they haven’t been achieved.

People very often ask me how long does it take to make a mug and they’re often taken aback when I say at least 2 weeks! Each piece is handled about 15 times. The casting and emptying of the mould, the impressing of a back stamp – which is important and needs to be applied at the right consistency when the clay still has a softness, but not so much that it will distort. Then there’s the application of the handle, making judgements about the placement and the precision of lining it up so it’s on straight, and then finally the fettling. A bisque firing follows – the first firing to drive off the moisture which makes the clay porous enough to take a glaze. The glazing of a piece: I find this a laborious process, as it’s lengthy pouring of glaze in and out of pieces and the cleaning of each piece until the excess is wiped away.

And, then after firing, finally the polishing. The outside of each piece isn’t glazed but it’s given a polish using abrasive diamond pads and papers to give each mug a pebbly smooth finish which is a more welcoming and tactile finale.

All good things take time… slipcasting is  definitely a slow movement.

* The DN team had to look this up. Thixtropy is defined as “the property of certain fluids and gels to become thinner when a constant force is applied, and then after reduction of the force, the viscosity recovers fully to the initial state in an appropriate period of time”.

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Laura Jacometti


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